Progress in Brain Research, Volume 278, the latest release in this ongoing serial, highlights new advances in the field with this new volume presenting interesting chapters on Treatment Resistant Depression overview in Adult, Treatment Resistant Depression in Children and Adolescents, Treatment Resistant Depression in Elderly, Headache and Treatment Resistant Depression, Insomnia and Treatment Resistant Depression, Tinnitus and Treatment Resistant Depression, Genetics of Antidepressant Response and Treatment Resistant Depression, Neuroinflammation via Gut-Microbiota-Brain Axis in Treatment Resistant Depression, Molecular Imaging findings for Treatment Resistant Depression, and more. Additional sections cover Functional MRI markers for Treatment Resistant Depression: Insights and Current Challenges, Medication Strategies for Treatment Resistant Depression, Brain Stimulation for Treatment Resistant Depression, Electroconvulsive Therapy for Treatment Resistant Depression, Next Generation Antidepressants with Novel Mechanisms for Treatment Resistant Depression, Psychological aspects and Psychotherapy for Treatment Resistant Depression, and Psychiatric Rehabilitation for Treatment Resistant Depression.